'exothermic' originates from Greek elements via New Latin/modern scientific coinage, specifically the Greek elements 'exo-' and 'thermē', where 'exo-' meant 'outside' and 'thermē' meant 'heat'.

'exothermic' changed from New Latin/modern scientific formation (from Greek elements, e.g. Greek 'exō' + 'thermē' or an earlier formation like 'exothermos') and eventually became the modern English word 'exothermic' in scientific usage in the 19th century.

Initially, it meant 'producing heat outward' (literally 'heat outwards'), and over time it became the technical term for processes or reactions that 'release heat' (i.e., are heat-producing).